Field of the Invention
The invention relates to low profile additives (“LPAs”) comprising branched polymers and methods for making the LPAs. The invention further concerns thermosettable resinous compositions, e.g., sheet molding compound (“SMC”) and bulk molding compounds (“BMC”), comprising the LPAs.
The Related Art
Polyester resins and vinyl ester resins may be used for thermosetting resin materials. Thermosetting unsaturated polyester resins are generally prepared by reacting dicarboxylic acids and anhydrides with glycols. Thermosetting vinyl ester resins are typically the reaction product of about equal amounts of a polyepoxide and an unsaturated monocarboxylic acid. When applied to thermosetting compositions, polyester and vinyl ester resins may be used in combination with fibrous reinforcement and inert fillers to manufacture composite structures, such as SMC and BMC. Such composite structures may be made by pre-mixing the resin, filler, fibrous reinforcement, and other additives to form the molding compound. For SMC, in particular, the resin, filler, initiator, and other additives can be mixed to form a paste, which is then sandwiched between two rolling polyethylene sheets, one of which is covered with chopped fiberglass reinforcements. Resinous paste wets the fiberglass and the whole mixture gets compacted while passing through a series of calendering rolls to form the SMC. BMC, on the other hand, is prepared by premixing resin, filler, and other additives to form a molding paste. Fibrous reinforcements are then added to the paste and mixed to form the BMC. The molding compound then can be formed into the desired shape and cured in a heated, matched metal die.
Chemical thickening of the relatively low viscosity liquid resin, e.g., with a Group II metal oxide or hydroxide and water, to form a high viscosity gel after the resin has been mixed with all other ingredients in the molding compound can be applied in the manufacture of SMC or BMC. This thickening or B-staging has several advantages. Unthickened molding compounds are sticky masses, which are difficult to handle. After B-staging, they are firm to very high viscosity solid-like gels whose surfaces are relatively dry, and may appear like leathery sheets, like in the case of SMC. In this form, the compound can be handled easily. During the molding operation, the molding compound flows within the die set to fill the die cavity. The increased viscosity of B-staged molding compounds inhibits segregation of the various components of the molding compound during flow and promotes compositional uniformity of the composite over the entire volume of the structure.
Thermosetting resins shrink in volume during ‘cure’. This shrinkage can cause warpage, internal stress build-up, dimensional changes, and poor surface cosmetics when reinforced composite objects are made from these thermosetting resins. Thermoplastic additives, typically referred to as LPAs, are known to reduce ‘cure shrinkage’ and associated problems. There are several types of polymers that may be used as LPAs, including saturated polyesters, polystyrene, poly(methylmethacrylate), poly(vinylacetate), and the like. Saturated polyester LPAs are generally linear polymers made primarily from aliphatic dibasic acids or anhydrides, like adipic acid or succinic anhydride, and diols, such as ethylene glycol, propylene glycol, or diethylene glycol. Linear LPAs made with high levels of aromatic diacids, like various phthalic diacids and anhydrides, and mixtures of symmetrical and asymmetrical diols, as well as those prepared with the addition of trimellitic anhydride, are also disclosed in the art.